While there have been substantial improvements in disk drive technology, the capacity and speed of disk drives is still far below that needed to provide on line access to large specialized libraries or databases. To store a library, such as all of the issued patents in the United States, requires a storage system capable of holding roughly a million megabytes (terabyte) of storage. A large law library would require a similar amount of storage.
Magnetic disks have access times of the order of ten milliseconds and capacities of less than 100 megabytes per surface. The cost of magnetic storage is about $1 a megabyte. Hence, a terabyte of storage can be provided only in systems with a very large number of users or a very high value database.
Optical disks provide improved data density and cost, but have significantly poorer performance than magnetic disk drives. A single-sided CD-ROM platter can provide 500 megabytes of storage at a cost of few dollars. Such systems are well suited to library applications in which the information stored thereon is not likely to change. Unfortunately, the access times are of the order of a 100 milliseconds and the data transfer rates are typically less than a megabyte per second. In addition to the cost of the platter, one must take into account the cost of the disk drive in assessing the total storage cost. Single platter disk drives have costs of the order of a few hundreds of dollars. If more than one platter is to be on-line at any given time, the user must either install multiple drives or purchase a disk changer. Hence, the actual cost of on-line storage is actually greater than $0.50 per megabyte. If a changer is used, the performance is also substantially reduced if access to several disks is required, since one must wait for the changer to unload the current platter and then load the next platter.
Broadly, it is the object of the present invention to provide an improved data storage system for use on data processing systems.
It is a further object of the present invention to provide a data storage system that has data densities of the order of those achievable with optical disk drives, but with improved access time.
It is a still further object of the present invention to provide a data storage system with a capacity much greater than that achievable on a single CD-ROM without requiring multiple disk drives or slow platter changers.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.